F125 is one of the hydrofluorocarbons (HFCs) used in the context of the substitution of chlorofluorocarbons (CFCs) and of hydrochlorofluorocarbons (HCFCs) which are already banned or in the course of being banned because of their harmful effect on the atmospheric ozone layer.
A number of access routes to F125 are known. The oldest consists in reacting tetrafluoroethylene with hydrofluoric acid in the presence of a catalyst. A particularly pure F125 is obtained in this way but with a cost which rules out any large-scale application in the field of commercial refrigeration or domestic air conditioning.
Recently proposed industrial processes all start with a starting material available in very large amounts at a very low price, perchloroethylene CCl.sub.2 .dbd.CCl.sub.2. A known industrial process consists in fluorinating perchloroethylene to F115 and in then hydrogenolysing the latter in the presence of a catalyst, in particular of a palladium catalyst. However, such a process is unsatisfactory because the conversion of the F115 is incomplete. The boiling points of F125 and of F115 are very similar and a simple distillation does not make it possible to separate the two components and thus to obtain pure F125.
The catalytic fluorination of perchloroethylene, in one or a number of stages, also produces not insignificant amounts (several thousands of ppm) of F115 in the crude F125 obtained.
Thus, whatever the process for the synthesis of F125 from perchloroethylene, the same problem is posed of the purification of the F125 in order to remove the F115 therefrom. This purification has thus formed the subject of much research and some solutions have already been disclosed.
U.S. Pat. No. 5,087,329 and Patent Applications EP 0,626,362, WO 92/21147 and EP 0,669,302 have provided the extractive distillation of the crude F125 using a solvent, such as, for example, an optionally halogenated hydrocarbon. However, this technique requires significant investments. It is likewise the case with the technique described in U.S. Pat. No. 5,346,595, which consists in subjecting the crude F125 to two successive distillations, one under superatmospheric pressure and the other under reduced pressure. It is not economically possible with these techniques to achieve very low F115 contents (less than 100 ppm).
Provision has also been made, in Patent EP 0,612,709, for the treatment of F125+F115 mixtures with hydrofluoric acid in the vapour phase over a catalyst, in order to convert the F115 to hexafluoroethane (F116). However, this process results in a pointless consumption of hydrofluoric acid and in the formation of a by-product with an economic value which is difficult to recover.
Application WO 94/02439 describes a process for the hydrogenolysis of the F125+F115 mixture over a hydrogenation catalyst at a temperature of between 300 and 600.degree. C. This technique is particularly effective but results in the production, as by-products, of 1,1,1,2-tetrafluoroethane (F134a) and especially of 1,1,1-trifluoroethane (F143a) which are difficult to separate by distillation.
The reactivity of CFCs with respect to hydrogen is known and Patent Application WO 91/05752 shows that it is possible to synthesize HFCs from CFCs and hydrogen in an empty reactor. However, this publication does not mention the purification of the HFCs and the operating conditions recommended for the synthesis do not result in an effective purification.